Generally, the capacity of a gear pump is determined by, for example, the tooth depth and tooth width of the gears, and the discharge flow rate is determined by the capacity and the rotational speed of the gears (i.e., the rotational speed of the pump). In a case where this gear pump is used as an oil pump for supplying lubricating oil into, for example, a vehicular engine, the capacity of the oil pump is set to supply oil at a sufficient amount for the lubrication even if the output of the engine as driving source is small, and therefore, the rotational speed of the pump is relatively low. On the other hand, as the output of the engine becomes larger, and thereby the rotational speed of the pump grows higher, the oil being supplied into the engine becomes excessive. In this condition, there is a possibility that a large driving force is consumed by the oil pump, which may lead to a power loss of the engined.
As a gear pump for solving this problem, there is a variable capacity type gear pump wherein, as the rotational speed of the pump becomes higher, both the drive gear and driven gear or one of them is correspondingly moved axially to reduce the meshing width of the gears and thereby reducing the capacity (refer, for example, to Japanese Laid-Open Patent Publication No. 2000-120559, and to Japanese Laid-Open Patent Publication No. S57-73880). Japanese Laid-Open Patent Publication No. 2000-120559 discloses a gear pump in which the driven gear is held axially by two side plates, with the support shaft of the driven gear being supported by these side plates. In this gear pump, a biasing force is provided on the back of one of the side plates while this biasing force is counteracted by a pressing force that is provided on the back of the other side plate, the pressing force being correspondent to the discharged fluid pressure. In this design, the driven gear, which is sandwiched between both the side plates, moves axially to the position where the pressing force is balanced against the biasing force. As a result, the meshing width of the driven gear with the drive gear varies in correspondence to the discharged fluid pressure.